The Environmental Protection Agency (EPA) issued the Clean Air Interstate Rule (CAIR) on Mar. 10, 2005, to reduce sulfur dioxide (SO2) and nitrogen oxide (NOX) emissions in the eastern United States. SO2 and NOx both contribute to the formation of fine particles (PM). In addition, NOx contributes to the formation of ground-level ozone. Fine particles and ozone are associated with thousands of premature deaths and illnesses each year. Additionally, these pollutants reduce visibility and damage sensitive ecosystems. Srivastava, R. K.; Hutson, N. D.; Martin, G. B.; Princiotta, F.; Staudt, J.; Control of Mercury Emissions from Coal-fired Electric Utility Boilers, Environ. Sci. Technol., 2006, 41, 1385.
Power plants are a major source of SO2 and NOx emissions. A number of power plants use wet flue gas desulfurization (FGD) technologies, particularly limestone-based wet scrubbers, to control SO2 emissions. In 2005, it was estimated that approximately one-third of U.S. coal-fired utility capacity employed some type of FGD technology. About 86% of those systems are wet-scrubber based and almost 70% of those wet scrubbers use a limestone process. Hutson, N. D.; Mercury Capture on Fly Ash and Sorbents: The Effects of Coal Properties and Combustion Conditions, Water, Air & Soil Poll: Focus, 2008, 8, 323.
Removal of SO2 from flue gas using a limestone process takes place in the scrubber as follows:SO2(g)+H2O→2H++SO32−  (1)2H++SO32−+CaCO3→CaSO3+CO2(g)+H2O   (2)
And, assuming there is an adequate supply of oxygen (air), the calcium sulfite may be then be oxidized to calcium sulfate (gypsum) as follows:CaSO3+½O2→CaSO4   (3)
Emissions of nitrogen oxides (NOX) in the U. S. are largely controlled by combustion controls (e.g., low-NOX burners, staged combustion) and selective catalytic reduction (SCR) or selective non-catalytic reduction (SNCR) technologies. As mentioned earlier, CAIR also mandates additional reductions in NOX emissions. As a result, it is predicted that the use of selective catalytic reduction (SCR) systems will significantly increase and will be used on slightly more than 50% of the total coal-firing power plants by 2020.
The EPA has also suggested a regulatory approach for the control of mercury (Hg) emissions from coal-fired power plants. Hg is a hazardous air pollutant (HAP) that poses serious health concerns due to its ability to bioaccumulate in the food chain—mainly in fish. There are both natural and anthropogenic sources of Hg emissions to the atmosphere. Of the anthropogenic sources, coal-fired utility boilers are believed to be the largest uncontrolled domestic source, accounting for about 48 tons in 1999. Srivastava, R. K.; Hutson, N. D.; Martin, G. B.; Princiotta, F.; Staudt, J.; Control of Mercury Emissions from Coal-fired Electric Utility Boilers, Environ. Sci. Technol., 2006, 41, 1385.
To date, wet-FGD scrubbers have been designed to specifically remove SO2 and NOx from flue gas. However, under the appropriate conditions, wet-FGD scrubbers can also remove soluble forms of other pollutants such as Hg. Proposals have involved Hg emission reductions through a combination of mercury-specific control technologies and “co-benefit” (or multi-pollutant) control technologies. Co-benefit reductions are those occurring via control technologies that are installed for other pollutants (e.g., PM, SO2, NOX) rather than specifically for mercury. However, deeper emission limits will likely require the use of additional Hg-specific control technologies. Wet-FGD scrubbers have the potential to provide a high level of Hg control because of their substantial gas-liquid mass transfer and the high solubility of most oxidized Hg compounds (such as HgCl2). However, this control is only effective for flue gas streams containing oxidized forms of Hg. The elemental form is not soluble and tends to pass through the wet scrubber. Srivastava, R. K.; Hutson, N. D.; Martin, G. B.; Princiotta, F.; Staudt, J.; Control of Mercury Emissions from Coal-fired Electric Utility Boilers, Environ. Sci. Technol., 2006, 41, 1385.
In general, facilities that burn coals with higher chlorine content (as often seen in eastern bituminous coals) tend to produce more oxidized Hg. Conversely, facilities that utilize coals with lower chlorine content (e.g., western sub-bituminous coals) tend to produce flue gases containing mostly elemental Hg. Additional equipment is needed to scrub flue gases that contain mostly elemental Hg.
Thus, while there are technologies available that successfully control the emission of an individual pollutant from a gas stream, a need exists for a cost-effective technology that can control multipollutant (e.g., SO2, NOX, Hg0 and Hg2+) vapor emissions from a gas stream obtained from of a stationary combustion source.